The present invention relates to a process for digital data retrieval in the presence of noise and distortions, as well as a device for implementing this process, the invention relating more particularly to receiving, in disturbed conditions, data transmitted in digital form in systems using as support medium a television video signal, these systems being known under the name "videotext".
It will be recalled that, in this kind of system, messages are transmitted in digital form using a binary code without return to zero, by means of data packages coming from one or more sources, each data package being preceded by a burst of identification and synchronization pulses formed by a succession of alternating 1s and 0s. This burst forms part, for the data package which follows, of a "heading" intended among other things for identifying the corresponding information source; its role is to provide a reference signal, at given frequency and phase, to which must be tuned a local clock of the receiver from which the decoding of the message contained in the data package is effected. More particularly, in the "videotest" system considered here, the data packages are inserted in television lines, each package being thus placed between two "line sync" signals usual in television, the technique used being that of time-based multiplexing of the digital signals with an analog image signal, which allows the existing frequency bands allocated to the transmission of television programs to be used. The data packages are thus transmitted over several successive lines or possibly over one isolated line among others containing simply the television image information. For a data package corresponding to a television line, there is inserted, after the usual "line sync" signal, the synchronization burst then the data package, the duration of said burst being short with respect to the data package. The transmission technique chosen requires a demodulation assembly comprising a television receiver associated with an interface system, adapted to be connected to a terminal on which the transmitted message is reconstituted. The interface is composed of a logic part capable of selecting the desired source and a demultiplexing part which retrieves from the video signal the digital information and reshapes it.
It is advisable to reconstitute here the digital data with a constant amplitude and a faithful spacing of the 1s and the 0s when the reception of the signal is disturbed. For this purpose a "slice" situated at level m is chopped in the video signal received so that, after chopping, a successive 1 and 0 are of the same width. To define the optimum chopping level m, one process consists in taking the average voltage value of the signal received during the synchronization burst, then in storing this average value, during the whole of the remaining duration of the digital data package, i.e. in the application more particularly considered here, until the end of the television line. The present invention is interested in establishing this average voltage value m and, more precisely, in defining the times between which the average value of the synchronization burst is measured (or better still: the value which tends towards chopping giving equality of the widths of the 1s and 0s).
Referring to FIG. 1 of the accompanying drawings, the problem posed is that of defining accurately the times shown at t3 and t5 between which the average value of the synchronization burst S is determined. t3 is the time when the measurement begins, this time coinciding preferably with the beginning of the first 1 of burst 2. t5 is the time when the measurement is finished and when the average voltage value m5 obtained is stored. The time chosen as origin, shown at as t0, is that when this storage is interrupted, the voltage then being brought to a "zero" reference value m0, for example the level of the black of the television signal.
According to one process already used, the zero-ing is effected by means of a signal f present between times t0 and t2. The beginning of this signal, i.e. time t0, corresponds to the beginning of the television "line sync" signal SL. The end of the signal f considered, i.e. the time t2, takes place around time t3 when the first rising front of the synchronization salvo S is due to arrive, and preferably a little before.
Another signal e, beginning after time t2 and finishing at a time t4, a little before the end of the burst and the beginning of the data D, initiates the system for measuring the average value m.
Unfortunately, it is very difficult to define with sufficient accuracy the duration and the position of the above-mentioned signals f and e, to bring them into coincidence with the beginning and the end of the emitted burst S, this especially in the case of disturbed reception. Now, it is important for the storage of the average value m to take place in any case before the end of the burst, and for the duration of the measurement of this average value, i.e. the duration of signal e to represent a sufficiently large fraction of the total duration of the burst.
The duration alone of signal e may be defined with sufficient accuracy, for example by means of a very stable and accurate monostable or better still, by counting the periods of the local clock used for reading the data D. However, the accurate definition of signal e involves the very exact knowledge of time t3 when burst S begins.
The beginning of signal e could be conditioned by time t2 of the end of signal f, itself in relation with the position of the "line sync" signal SL, but time t2 is thus defined with very low accuracy with respect to the real time t3 when burst S appears.
One idea for solving this problem would be to cause signal e to begin in relation with the detection of the appearance of the first rising front of synchronization burst S, by carrying out this detection on the chopped signal (in the sense which was defined above) so as to be as independent as possible of the noise. However, since the average value was brought to a zero level m0 before time t3, the chopped signal DM would contain all the noise, and it would be possible to retrieve therefrom the first front of the burst.
If the amplitude of the signal forming the burst is a little variable, this problem may be simply resolved by imposing on value m, after the average value has been taken, a fixed non zero threshold value, corresponding substantially to half the amplitude provided for the 1s of the burst. Unfortunately, in disturbed reception conditions, the amplitude of the burst is very variable and can only be known after its appearance, which makes the fixing of such a threshold value difficult.
The present invention aims at removing this difficulty.